As for known techniques for protecting automobile wheels from being stolen by specially profiling hub nuts to be applied to hub bolts, there are Japanese Jikkai Sho 58-19612 and Sho 61-44016, which are thought to most closely resemble the present invention.
In said Japanese Jikkai Sho 58-119612, however, even if the outer peripheral surface 4 of a box nut body 1 is tapered, it is still round in shape and, moreover, engaging grooves 6 cut into the round outer peripheral surface 4 present a convex-concave pattern as a whole, wherein the concave grooves are diametrically opposed to each other, so that the box nut can be gripped as by commercially available pliers, thus being still in danger of being turned.
Further, said box nut body 1 has an engaging recessed portion 5 exposed on its front end and the inner peripheral surface of said engaging recessed portion 5 is also an angular type in which it recesses at diametrically opposite positions, so that it is in danger of being turned by a commonplace bifurcated fork being inserted therein.
Further, when said box nut body 1 is to be operated for turning by a genuine cap 2, as is obvious from FIG. 2, it has to be turned by engaging the engaging recessed portion 5 and the engaging grooves 6 of the box nut body 1 with the engaging convex portions 8 and 9 of the cap 2.
That is, since the outer peripheral surface of the box nut body 1 is circular, it is impossible to apply the surface pressure of the cap 2 (the torque produced by the wrench) to the whole of the box nut body 1 during the turning operation; the surface pressure locally acts only on said engaged surfaces.
As a result, it is difficult to correctly smoothly turn the box nut body 1 even by the use of the genuine cap 2, often resulting in deformation or chipping of said engaged portions, presenting a problem that such operation tending to lower the durability of the box nut body 1.
On the other hand, since the wheel nut 1 disclosed in Japanese Jikkai Sho 61-44016 has a polygonal engaging portion 3, 13, even if it is shaped as a tapered surface 3a, 13a, a commercially available wrench can be directly engaged with it, so that it is in danger of being turned without having to take the trouble of using the turning member 6.
Concerning this point, in the case of the embodiment shown in FIG. 3, although the polygonal engaging portion 13 is splined like a spline shaft, the grooves are diametrically opposed to each other, so that it has to be said that there is still the possibility of the wheel nut being turned as by commonplace pliers or wrench.
Further, in the case of the cut pattern of said polygonal engaging portion 3, 13, it would be difficult to prepare various forms of said steal preventing wheel nut 1 as so-called different-keyed configurations in abundance.